The present invention relates to electrophotography and more particularly, to a developing material for use in a dry process method for developing electrostatic latent images, which developing material is also effective when employed in a method for developing a magnetizable toner which can be transferred onto plain copy paper through an electric field established by corona discharge.
Commonly, in the known dry process electrophotographic developing methods which have already been put into practical application, there is employed a developing material produced by mixing particles called carriers, for example, of iron particles or glass beads with non-magnetizable fine particles called toners, which developing material is mainly employed in the conventional two component developing methods, for example, the magnetic brush developing method, cascade developing method, etc.
The dry process two component developing methods as described above, however, have various problems particularly related to the carrier in that such carrier only serves in charging and transporting the non-magnetizable toner particles or as the developing electrode (in the case of the magnetic brush developing method) and is not directly engaged in the developing itself. Thus it is not consumed during each copying, and it gradually deteriorates with the increase in the number of copies made, generally making it necessary to be replaced after a predetermined number of copies has been made. Furthermore, particularly when the particle size of the carrier is too small or excess carrier is employed, the carrier may adhere onto the surface of the photosensitive member or photoreceptor in some cases, thus adversely affecting the quality of the copied images. Therefore, control of particle sizes and stabilization of the mixing ratio of carrier to toner are required, but such countermeasures are not desirable, since the size of the copying apparatus and developing device tend to be increased thereby.
Moreover, since the diameter of the carrier particle can not be made excessively small due to the above fact, an increase in the surface of such carrier particle is inevitably limited, and depending on the mixing ratio of the carrier to the non-magnetizable toner, there are cases where uneven charging for the non-magnetizable toner may take place.
In order to overcome the disadvantages inherent in the two component developing method as described above, it is conventional to employ a one component developing method employing magnetizable toner particles, and a direct type copying apparatus, i.e., copying apparatus which uses a photosensitive paper employing photosensitive material, without effecting transfer and which is based on the one component developing method. Meanwhile, although various attempts have also been made to apply the one component developing method to the copying apparatuses of transfer type, there are difficult problems to be solved related to physical properties in the developing and transfer in that conditions contrary to each other i.e., electrical conductivity during developing and electrical insulation during transfer are simultaneously required. More specifically, while the devoping is successful in the case of the electrically conductive, magnetizable toner particles having high electrical conductivity, there is a disadvantage such that during electric field transfer onto plain copy paper, the polarity of the magnetizable toner is varied due to injection of charge thereto from the copy paper, thus resulting in the so-called Blow-off phenomenon in which the toner once transferred onto the copy paper again leaves the same copy paper to cause non-uniform density and turbulence in the copied images.
For eliminating the undesirable non-uniform density and turbulence in the copied images as described above, there have conventionally been proposed various arrangements such as employment of electrically insulated copy paper (disclosed, for example, in Japanese Laid Open Patent Application Tokkaisho No. 50/90336, and Japanese Patent Publication Tokkosho No. 49/11576), pre-heating of copy paper (disclosed, for example, in Japanese Laid Open Patent Application Tokkaisho No. 50/43936), and uniform exposure of photosensitive surface to light before or during transfer (disclosed, for example, in Japanese Laid Open Patent Applications Tokkaisho No. 51/26044 and Tokkaisho No. 51/96332) etc., none of which is, however, related to improvement of the one component developing method.
On the contrary, the electrically insulating magnetizable toner particles have problems related to developing. More specifically, since such electrically insulating magnetizable toner particles are not sufficiently stable in charging, the developed images tend to be undesirably soiled, and for eliminating such disadvantages, auxiliary means, for example, means for subjecting the electrically insulating magnetizable toner to corona charging within the developing apparatus is required as disclosed in Japanese Laid Open Patent Application Tokkaisho No. 50/117432, thus resulting in complication in the structure of the developing apparatus.
Although a developing method employing a magnetizable toner having properaties intermediate the electrically conductive magnetizable toner and the electrically insulating magnetizable toner has also been conventionally proposed, for example, in Japanese Laid Open Patent Application Tokkaisho 50/92137, it is quite doubtful whether such magnetizable toner can satisfactorily provide the properties of the electrically conductive magnetizable toner and electrically insulating magnetizable toner, while problems arise in such magnetizable toner from the viewpoint of difficulty in manufacture thereof, stability under various temperatures and humidity conditions, etc.